Science Briefs

Plague and Volcanoes

April 2000

Plague! The word conjures up kaleidoscopic images of medieval pestilence and modern media hype. As a specific disease, however, its meaning is now confined to the infectious, and often fatal, illness caused by the bacterium Pasteurella pestis. Common carriers of true plague are rat and human fleas and the human louse.

Louis Pasteur in the 19th century suggested that the cyclical agent of plague, whatever it was, might lie very low until suddenly reactivated by a change in the climate. As early as the 16th century, the poet Thomas Nashe in the midst of a British plague lamented, "From winter, plague and pestilence, good Lord, deliver us!" In fact, it is now recognized that wet, chilly weather stimulates the reproduction of the deadly bacterium that causes bubonic and pneumonic plague. Yet, the origin of the historical cycles of plague has remained obscure.

By statistically examining the occurrences of widespread plague that have been recorded throughout Western history, a curious coincidence emerges. A large number of these pandemics occurred shortly after a volcano underwent a huge eruption, according to GISS scientist Richard Stothers.

Mount Vesuvius in repose. A potential trigger of future plague?

Volcanoes have been known to spew sulfurous gases into the stratosphere, where the sulfur combines with water vapor to form sulfuric acid aerosols. These aerosols screen out some of the radiation coming from the Sun and so cool off the Earth's surface. They also change the circulation patterns of the atmosphere, such that the northern jet stream moves farther south, bringing cold polar air down into midlatitudes, where Europe and the Middle East lie. These areas are the main reporting regions throughout most of Western history.

Large volcanic eruptions that took place in early times beyond the Mediterranean basin have been detected, indirectly, by the heavy fallout of stratospheric aerosols onto the polar ice caps. There the aerosols are incorporated into the year's snow accumulation, which later becomes compacted into ice and, as such, remains preserved like a fossil. After drilling an ice core, scientists count the annual layers downward to determine the ages of the layers with an exceptionally high sulfur content. By this means, or from direct visual reports of eruptions, we know that the seven largest volcanic eruptions of roughly the past two millennia occurred in 44 BC and in AD 536, 626, 934, 1258, 1783, and 1815.

All of these eruptions were followed by stratospheric dry fogs that dimmed the Sun's light, chilled the atmosphere, and led to an increase in the amount of precipitation. In each case, many food crops failed and a fatal pandemic, originating from a focus in Asia or Africa, spread throughout the Mediterranean area within one to five years after the eruption. It is believed that in at least five instances the contagion responsible for the mass mortality was true plague.

Not all plague pandemics, however, can be traced to large volcanic eruptions. This is not surprising, since prolonged wet, chilly weather can arise from other natural causes. Although the Black Death of 1348 in Europe is a case in point, this major exception is practically unique. In any event, plague has still not been eradicated from the world, and we must bear in mind that volcanic eruptions have not been, either.